German mathematical terms like "Nullstellensatz" There are quite a few german mathematical theorems or notions which usually are not translated into other languages. For example,
Nullstellensatz, Hauptvermutung, Freiheitssatz, Eigenvector (the "Eigen" part), Verschiebung.
For me, as a German, this is quite entertaining. Do you know other examples? Please one per answer, please give a reference for the term or a short explanation of what it means.
It would be great to see an explanation why there is no translation.
EDIT: Some more examples can be found at Wikipedia: Ansatz, Entscheidungsproblem, Grossencharakter, Hauptmodul, Möbius band, quadratfrei, Stützgerade, Vierergruppe, Nebentype.
 A: There's Soergel's Endomorphismensatz and Struktursatz.
A: The notation  $G_\delta$ is from German, $G$ for Gebiet, and $\delta$ for Durchschnitt. Strangely enough, the notation for the co-sets, $F_\sigma$, is from French, fermé and somme.
A: Verschränkungsoperator is the (perhaps even original) german version of "intertwiner" which I really like. But I've not seen that very much ;)
A: Viergeflechte, the original German name for 2-bridge knots, still occasionally used in an English context. In his Mathematical Review of Schubert's 1956 paper "Knoten mit 2 Bruecken" Fox explicitly notes that "Viergeflecht" is untranslatable.
A: Zahlbericht (Hilbert), Klassenkörperbericht (Hasse),
Das blaue Hasse (Zahlentheorie, Akademie-Verlag, Berlin).
A: The practice to use Gothic letters sometimes for ideals ($\mathfrak{a}$, $\mathfrak{b}$, ...) and often for Lie algebras ($\mathfrak{g}$, $\mathfrak{h}$, ..) seems to be of German origin.
Also to use the lesser known "kernel" instead of the better known "core" seems to stem from the German "Kern".
A: I would like to mention a handful of examples that may be considered passé nowadays, but were prominent at some point in time.


*

*Schlicht: I dare to address this once again because I consider that the feedback in the comments below Gottfried's entry is kind of misleading. About this one, Boas says that (see [1, page 97]):



... When I was an undergraduate, there was no regular colloquium at
  Harvard, but there was a Mathematical Club, whose meetings were
  regularly attended by faculty. Once somebody gave a talk on schlicht
  functions. After the talk, Julian Lowell Coolidge asked plaintively
  whether there was an English word for 'schlicht'. Osgood replied,
  "Well, you could call them univalent functions, and everybody would
  know that you meant 'schlicht'". You need to know that Osgood had been
  trained in Germany, wrote his treatise on complex analysis in German,
  and was apt to tell German jokes to his classes.

It has to be noted that in practice univalent and schlicht are not perfect synonyms. For instance, on Function theory of one complex variable by Greene and Krantz, we can read this (my emphasis):

A holomorphic function $f$ on the unit disc $D$ is usually called
  schlicht if $f$ is one-to-one. We are interested in such one-to-one $f$ that satisfy the normalizations $f(0)=0$ and $f^{\prime}(0)=1.$ In
  what follows, we restrict the word schlicht to mean one-to-one with
  these normalizations.

What is more, several online sources include right from the start those normalizations in their definition of schlicht, e.g., planetmath.org, Wikipedia, and Wolfram MathWorld.


*

*Aussonderungsaxiom: Of all axioms of Zermelo, I have noticed that, for some godforsaken reason, in some books/papers written in English (and even in Spanish) this one is (or was) occasionally called by its German name.

*Limes: That's right... It was not a typo in Ahlfors's text on Complex Analysis. I recently came across this one in another book, but I just can't recall which one it was.
EDIT: According to Gerald Edgar "limes" is a Latin word. Yet, I will leave it here because I believe that it is a loan word in German which made it to other languages due to the influence of treatises written originally in German. 


*

*Drehstreckung: Tristan Needham recalls this one when he apologizes for the coinage of the term 'amplitwist'. More specifically, he writes



To the expert reader I would like to apologize for having invented the
  word 'amplitwist' ... as a synonym (more or less) for 'derivative', as
  well the component terms 'amplification' and 'twist'. I can only say
  that the need for some such terminology was forced on me in the
  classroom: if you try teaching the ideas in this book without using
  such language, I think you will quickly discover what I mean!
  Incidentally, a precedence argument in defence (sic) of 'amplitwist'
  might be that a similar term was coined by the older German school of
  Klein, Bieberbach, et al. They spoke of 'eine Drehstreckung', from
  'drehen' (to twist) and 'strecken' (to stretch).

Last but not least, in several works of old (z.B., Perron's Die Lehre von den Kettenbrüchen, Knopp's Theory and Application of Infinite series, Khinchin's Continued Fractions), there appears the following notation for general continued fractions:
$$\underset{j=1}{\overset{\infty}{\LARGE\mathrm K}}\frac{a_j}{b_j} = \cfrac{a_1}{b_1+\cfrac{a_2}{b_2+\cfrac{a_3}{b_3+\ddots}}}.$$
Guess what the $\mathrm{K}$ stands for...
References
[1] Lion Hunting & Other Mathematical Pursuits: A Collection of Mathematics, Verse and Stories by Ralph P. Boas Jr.
A: Ansatz. Although I suppose it is used more in physics than in mathematics. I don't know why the translation is not used often, but I guess it has to do something with the fact that in the beginning of the 20th century German was used much more than English in the scientific literature, I believe.
A: Einheit = word for unit in algebra. Hence, some use the notation $e\in G$ to denote the element of a group such that $ex = xe = x , \forall x \in G$. Unit is the appropriate translation, yet some algebraist still use the letter $e$ to denote the identity element in a group. 
A: Spiegelungssatz. The meaning of this theorem is briefly
discussed in the article:  Iwasawa theory and $p$-adic
deformations of motives [MR1265554 (95i:11053)] by Ralph
Greenberg.
Let $p$ be an odd prime, and
$K_\infty=\mathbf{Q}(\mu_{p^\infty})$. Let $L_\infty$ denote the
maximal unramified abelian pro-$p$ extension of $K_\infty$, and
$M_\infty$ the maximal abelian pro-$p$-extension of $K_\infty$
that is unramified outside the primes above $p$. Let
$Y_\infty={\rm Gal}(L_\infty/K_\infty)$ and $X_\infty={\rm
Gal}(M_\infty/K_\infty)$. We can decompose ${\rm
Gal}(K_\infty/\mathbb{Q})\cong\Delta\times\Gamma$, where
$\Delta={\rm Gal}(\mathbf{Q}(\mu_p)/\mathbf{Q})$ and
$\Gamma\cong\mathbf{Z_p}$. Both $Y_\infty$ and $X_\infty$ have a
natural structure of $\Lambda$-modules
($\Lambda=\mathbf{Z_p}[[\Gamma]]$) coming from the action of ${\rm
Gal}(K_\infty/\mathbf{Q})$ by inner automorphisms. The latter
action gives in particular an action of $\Delta$, and hence we can
decompose $Y_\infty=\bigoplus_{i=0}^{p-2}Y_\infty^{\omega^i}$ and
$X_\infty=\bigoplus_{j=0}^{p-2}X_\infty^{\omega^j}$ as
$\Lambda$-modules, where the superscript denotes isotypical
component under the action of $\Delta$, and
$\omega:\Delta\rightarrow\mu_{p-1}$ denotes the mod $p$ cyclotomic
character. The spliegelungsatz is then described by Greenberg in
loc. cit. as an argument using Kummer theory and class field
theory that allows to relate the structures of
$X_\infty^{\omega^j}$ and $Y_\infty^{\omega^i}$ for $i+j\equiv
1\pmod{p-1}$ as $\Lambda$-modules.
A: In topology the separation axioms  $T_0$ , $T_1$ .. etc, where the $T$ stands for Trennungsaxiom
A: This is an answer to the part of the question about why these terms are not translated into English. The reason is that  words such as "nullstellensatz",  "Schadenfreude" and so on that you mistakenly think are German are in fact perfectly good English words and so do not need translation. (Look up  Schadenfreude in the Oxford English Dictionary if you do not believe it is an English word, though they have not yet caught up with nullstellensatz.) The point is that unlike languages such as French and German that try to remain pure, English has been happily looting terms  from other languages for centuries, and the only difference between "nullstellensatz" and "house" is that "house" was stolen so long ago that we have forgotten about it. 
A: And what about the Wiedersehen metric?
A: The following theorem is known as Kugelsatz:
Let $X$ be an open set in $\mathbb{C}^n, \quad n \geq 2$ and $K \subset X$ a compact subset such that $X\setminus K$ is connected. Then the restriction map $\rho: \mathcal{O}(X) \mapsto \mathcal{O}(X \setminus K)$ is an isomorphism of $\mathbb{C}$-algebras (this version after: Volker Scheideman, Introduction to Complex Analysis in Several Variables, Birkhäuser 2005).
The first result of this kind is due to Hartogs, with $X$ and $K$ being concentric euclidean balls, hence the name (Kugel=ball). Many textbooks in several complex variables have been written by German-speaking authors (Grauert+Fritzsche, Kaup brothers are other examples), so the German name stuck even in the English version. The theorem is also referred to as "tomato can principle".
A: One that is similar in spirit "eigenvalue" in that it mixes the two languages is 
$$
\text{umkehr map}
$$
A: The Hegelian term Aufhebung has been appropriated by Lawvere to refer to relations between essential subtoposes of a cohesive topos, with a view to doing abstract homotopy theory. See the nLab for more. 
A: All this should be compiled in a Festschrift.
A: This is a notation rather than a term, but the wide use of the letter $K$ to denote a field in Algebra refers to the German word Körper.
A: Jugendtraum (Kronecker).
A: Bew (short for beweisbar, introduced by Gödel's incompleteness paper) is still used as a provability predicate in some mathematical logic papers.
In physics and other subjects (not so much in math) we hear about plenty of Gedankenexperiments.
Don't forget Hilbert's Satz 90, anomalous because of the "90" and not just the "Satz".
There are also French words like étale cohomology.
A: There is Ahlfor's scheibensatz in complex function theory, which is a generalization of  Ahlfors five islands theorem
A: If you think of the symbols, you can also see Gothic, alternatively called German, letters.
Also, in algebraic topology, it is common to show the cycles by $Z$, which is the first letter of Zykel.
Also, many words that are Latin or Greek, in terms of the ingredients, were first coined and used in German, like Topologie which used to be called Analyse Situs.
It was common to show curvature by $K$, which stands for Krummung. Also, it was common to show a domain by B, for Bereiche. Or in riemannian geometry, the metric tensor is represented by $g$, which stands for Gravit\"at
Also, Faltung used to be common in English before the word convolution took over.
I can also add Umlaufssatz in the differential geometry of surfaces.
There are so many more...
A: There's a kind of combinatorial design called a gerechte design - essentially it's a Latin square with additional block constraints.  (I gather there's been a fad in recent years for newspapers to print partial gerechte designs of a certain kind for readers to complete.)  As a technical term, the word comes from the following paper:
W. U. Behrens (1956). Feldversuchsanordnungen mit verbessertem Ausgleich der Bodenunterschiede. Zeitschrift für Landwirtschaftliches Versuchs- und Untersuchungswesen, 2, 176–193.
Behrens' gerechte designs were 'fair' in how they apportioned plots of land to different treatments in an agricultural trial.
A: There is also the Quermassintegral (mixed volumes of the form $V(K,K,\ldots,B,B)$ where $B$ is the unit ball, see Wikipedia), which I'm not even sure is German (not a lot of Qs in German usually).
A: Schubfachprinzip ("drawer principle" or "shelf principle" or  "Dirichlet's box principle"). It is now easy to guess we are talking about P-H P.
A: The word idele ultimately comes from the abbreviation
"id. ele." for ideales Element.
A: An indirect answer:
Klein bottle
which has probably started out as:
Kleinsche Fläche (=Klein surface)
Kleinsche Flache (lost umlaut in English print)
Klein bottle (translation of Flasche instead of Flache) 
A: Verlagerung.  Sometimes translated as the transfer.
A: Nebentypus, Positivstellensatz.
A: Ganzstellensatz.
A: Zusammenstellung. Means "compilation" or "survey". Can be used in the first section of a paper, as one starts compiling "preliminary facts" to refer to later in the paper. That's the way I've seen it used in a paper by Raoul Bott.   
A: I believe Albrecht Frölich uses the german term beweis, instead of the english proof, in his chapter of the classic "Algebraic number theory". (EDIT: In my original version, I translated beweis to example. I shouldn't trust my poor knowledge of German... )
A: Umkehr map (pushforward map).
A: Apparently the term K-theory comes from the German word "Klasse", according to Wikipedia and  http://arxiv.org/abs/math/0602082
A: Here's another one: Hauptvermutung
A: In GR (and other branches of mathematical physics) one uses vierbein (tetrad) and more often these days also vielbein, for local orthonormal frames in a (pseudo-)riemannian manifold.
A: Plastikstufe = a certain higher dimensional analogue of an overtwisted disk in contact geometry.  This is not a real German word.  It is a compound of the German words for "plastic" and "step", but this does not have any obvious relevance to its mathematical meaning.  There is a funny story about where this word came from which however is not appropriate for this forum.
A: The Verschiebung morphism.
A: Größencharakter.
http://en.wikipedia.org/wiki/Hecke_character
A: Führerdiskriminantenproduktformel.  
A: Gentzen's Hauptsatz (cut elimination theorem) : This is a fundamental result in structural proof theory, and is at the heart of Gentzen's consistency proof of elementary number theory.  It is very funny that the word literally means "main theorem," with no reference to the subject domain, yet it is standard in logic in English to use just the word "Hauptsatz" to refer to this (family of) theorem(s) in proof theory.
A: Zugzwang - a sort of Nash Equilibrium. This terminology is specifically used in Chess.
A: I've seen schlicht-function for functions $f(x)=x +a x^2 + b x^3 + \cdots$ for powerseries without constant term and $f'(0)=1 $. But I do not really know, whether this is really the german word schlicht (=simple) or only some coincidence.
A: Die Vierergruppe.
A: deck transformation?
A: "Urelement" is used in set theory as a fancy name for an atom, i.e., something that can be a member of a set but is not itself a set.   
A: Some famous book published in about 1950 says that for lack of an English word for the concept the word Faltung is used.  In recent decades, the adapted Latin word convolution has served.
Paul Halmos tried unsuccessfully to expunge the words eigenvector and eigenvalue from the language, using the terms proper vector and proper value in his book Finite-dimensional Vector Spaces.
A: Stufe (=level) of a non-real field (wikipedia.de). It is the least number of squares $a_i^2$ such that $\sum_i a_i^2 = -1$, $\infty$ if no such sum exists.
In this paper, the level of a subgroup of $SL_2(\mathcal{O})$ is defined ($\mathcal{O}$ a number field), as the generalisation of the stufe of a field, so the term has been translated, but only in a shift of context.
To pick a random paper, try The stufe of number fields.
A: In "Functional Analysis" by Kosaku Yosida he denotes the closure of a set $M$ by $M^a$.
He explains that it is a shortcut from German abgeschlossene Hulle.
A: Gleichverteilungssatz, which refers to both H. Weyl's result in complex analysis and ergodic theory ([1] §5, pp.18-19) and to several theorems in statistical physics
(Boltzmann's,... etc.).
[1] Heins, Maurice (1962), Selected Topics in the Theory of Functions of a Complex Variable, New York: Holt, Rinehart and Winston, Athena Series. Selected Topics in Mathematics, xi+160, MR0162913, Zbl 1226.30001.
A: "schlichtartig" refers to a surface on which every simple closed curve which separates locally, also separates globally.  Hence it means roughly "planar".  This is used in the conformal mapping theory of Riemann surfaces.  Introduction to Riemann Surfaces, Springer, p. 91.  I know only a little German but it seems to translate something like "simply behaved"?
A: Anzahl-theorems is  one I have recently read  in Wan's book on classical groups.
A: Kegelspitzen. There are directed complete orders equipped with a convex structure such that all relevant operations are (Scott) continuous. Introduced by Klaus Keimel and Gordon Plotkin in https://arxiv.org/abs/1612.01005
It literally means "tip of a cone"; the motivation is that you would obtain a Kegelspitze by considering a cone and cutting of its tip. I guess that because the English description would be three words instead of a single one, the authors chose for the German translation, probably also because one of the authors was German.
A: The $\int$ symbol is a german S introduced by Leibniz and stands for Summe (Sum)
A: It's early in the morning, so maybe I missed it in the answers above, but, if we're including symbols, then the obvious example is $\mathbb{Z}$, the integers, or zahlen!  
Ooops!  It is early in the morning... I see that Roland noted that the symbol for the integers (which I also can't seem to get to process properly) just a few comments above.
A: In Swedish, a field is called a 'kropp', a body. This of course from the German word Körper.
